1. Technical Field
The present invention relates to power supply apparatuses, and more particularly, to a power supply apparatus for a DC-operating high-voltage discharge lamp.
2. Background Art
High-voltage discharge lamps that are turned on by application of DC voltage and used as lamps for projectors, for example, are available. Although the air pressure inside a high-voltage discharge lamp is relatively low when the lamp is not turned on (at a normal temperature), the air pressure rises in accordance with rising temperature after discharge starts. Since discharge is more likely to occur when the air pressure inside a discharge lamp is lower, discharge is likely to occur and the resistance of the discharge lamp, as a load, is low when discharge starts. In contrast, a discharge is relatively less likely to occur and the resistance of the discharge lamp, as a load, becomes higher after the temperature rises due to turning on of the discharge lamp. However, since resistance values of loads are not always constant, a power supply apparatus that controls the load power to be constant is preferably used for a high-voltage discharge lamp.
A power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-189602 (Patent Document 1) is available. The power supply apparatus described in Patent Document 1 is used to implement zero-voltage switching (ZVS) of switch elements in a step-down or step-up type chopper circuit, and the power supply apparatus includes a resonance coil, a capacitor serving as a constant voltage source, and a second switch element in addition to basic components constituting the chopper circuit, such as a first switch element, a choke coil, and a rectifier diode. However, the power supply apparatus disclosed in Patent Document 1 is not specifically intended to be used for turning on a discharge lamp.
FIG. 3 is a circuit diagram of a power supply apparatus according to a first embodiment of Patent Document 1. In FIG. 3, a power supply apparatus 10 is based on a step-down chopper circuit, and includes a direct-current power supply Vin, a rectifier diode D1, a choke coil L1, a MOSFET Q1 serving as a first switch element, and a smoothing capacitor C1 as basic elements of the chopper circuit. The power supply apparatus 10 also includes a diode D2 serving as a first diode, and capacitors C2 and C4. The diode D2 is a body diode for the MOSFET Q1, and the capacitor C2 defines a junction capacitance, that is, the parallel capacitance between the drain and source of the MOSFET Q1. The capacitor C4 defines junction capacitance, that is, the parallel capacitance between the anode and cathode of the rectifier diode D1.
The power supply apparatus 10 further includes a MOSFET Q2 serving as a second switch element, a diode D3 serving as a second diode, a capacitor C3, a resonance coil L2, a capacitor C5 serving as a first capacitor that is a constant voltage source, a diode D4 serving as a third diode, a resistor R1 for current detection, and a control circuit 11. The diode D3 is a body diode of the MOSFET Q2 and the capacitor C3 defines a junction capacitance, that is, the parallel capacitance between the drain and source of the MOSFET Q2.
The drain serving as a first end of the MOSFET Q1 is connected to a first end of the direct-current power supply Vin and the source of the MOSFET Q1 serving as a second end of the MOSFET Q1 is connected to a first end of the choke coil L1 through the resonance coil L2. A second end of the direct-current power supply Vin is grounded. A second end of the choke coil L1 is connected to a first output terminal Po. The cathode of the rectifier diode D1 serving as a first end of the rectifier diode D1 is connected to the first end of the choke coil L1, and the anode of the rectifier diode D1 serving as a second end of the rectifier diode D1 is connected to the second end of the direct-current power supply Vin. The smoothing capacitor C1 is connected between the first output terminal Po and a second output terminal Po. A first end of a series circuit including the capacitor C5 and the MOSFET Q2 is connected to a node connecting the MOSFET Q1 and the resonance coil L2, and a second end of the series circuit is connected to the second end of the direct-current power supply Vin. A resistor R1 is connected between the first end of the smoothing capacitor C1, that is, the second output terminal Po, and the second end of the direct-current power supply Vin. The diode D4 is connected in parallel to a series circuit including the MOSFET Q1 and the resonance coil L2. The control circuit 11 is connected to the first and second output terminals Po, both ends of the resistor R1, a gate serving as a control terminal of the MOSFET Q1, and a gate serving as a control terminal of the MOSFET Q2. The order in which the capacitor C5 and the MOSFET Q2 are connected may be reversed so long as they are connected in series.
In the power supply apparatus 10 having the above configuration, the control circuit 11 detects an output voltage Vout. Since output current flows through the resistor R1, the control circuit 11 detects the output current from a voltage across the resistor R1. The MOSFETs Q1 and Q2 are alternately turned on and off with a dead time therebetween in which both MOSFETs are turned off so that the output voltage or the output power becomes constant. The frequency of MOSFET switching follows the frequency of an oscillation circuit contained in the control circuit 11, and stabilization of the output voltage or the output power is performed by changing the ON-OFF duty cycle of each switch element, that is, by PWM control.
The control circuit 11 has an overcurrent protection circuit for temporarily blocking the oscillation to stop the switching (preventing the MOSFET Q1 from being turned on) to solve the problem when the output current becomes an overcurrent.
A case in which the power supply apparatus 10 is used to turn on a high-voltage discharge lamp will be considered. It is assumed that the choke coil L1 has an inductance of 1 mH, the resonance coil L2 has an inductance of 30 μH, the smoothing capacitor C1 has a capacitance of 0.47 μF, and the capacitor C5 has a capacitance of 0.47 μF. It is also assumed that the capacitors C2, C3, and C4 have capacitances in the range of dozens to hundreds of pF. An input voltage Vin is assumed to be 370 V, and a predetermined output voltage Vout on startup of the power supply apparatus 10 is assumed to be 280 V. The switching frequency of the switch elements according to the oscillation circuit included in the control circuit 11 is assumed to be 100 kHz.
In the power supply apparatus 10, the control circuit 11 operates so that the output voltage becomes constant on startup of the power supply apparatus. When the high-voltage discharge lamp is to be turned on, 17 kV is added to an output voltage of 280 V by an igniter provided between the power supply apparatus and the high-voltage discharge lamp, and discharge is started by applying the combined voltage to the high-voltage discharge lamp. The power supply apparatus operates in a light-load state since no current flows through the high-voltage discharge lamp for a few hundred milliseconds before discharge starts.
Once discharge of the high-voltage discharge lamp is started, the output of the power supply apparatus 10 is directly applied to the high-voltage discharge lamp. Since the resistance of the high-voltage discharge lamp as a load is low when the temperature is low immediately after the discharge is started, a large current attempts to flow. In actuality, however, the overcurrent protection circuit is activated and the current is limited to, for example, about 4 A. At this time, the voltage across the high-voltage discharge lamp is about 10 V.
Thereafter, the temperature of the high-voltage discharge lamp rises within several tens of seconds, the resistance of the discharge lamp as a load becomes high, and the discharge state becomes stable. However, even if the discharge becomes stable, the load resistance value of the high-voltage discharge lamp is not always stable. Therefore, as the load resistance value changes, the power supply apparatus 10 performs constant power control so that the load power becomes constant, for example, at 200 W. Specifically, the output voltage of the power supply apparatus 10 changes, for example, within the range of approximately several tens of volts to a hundred and several tens of volts.
As described above, in the case where the power supply apparatus 10 is used for turning on the high-voltage discharge lamp, the power supply apparatus 10 operates with a light load since negligible load current flows when the power supply apparatus 10 starts up. In the power supply apparatus disclosed in Patent Document 1, there seems to be no apparent problem since an operation of the circuit is described assuming that there is a normal load. However when the load is light at the starting of discharge of the high-voltage discharge lamp, there is a problem, since the operation of the circuit is different from a normal operation.
First, the capacitor C5 is charged by the power supply voltage Vin so that the MOSFET Q2 side becomes negative when the load is light. The charging voltage Vx of the capacitor C5 at this time may reach the output voltage (Vx=−280 V) at maximum. In contrast, when the normal load value is applied, the capacitor C5 is charged at a substantially constant voltage Vx so that the MOSFET Q2 side is positive. In this case, Vx is in the range of 10 V to 20 V. Charging the capacitor C5 at a predetermined voltage Vx makes the voltage Va at the node of the MOSFET Q1 and the resonance coil L2 to be −Vx when the MOSFET Q2 is ON, and the direction of the current ib flowing through the resonance coil L2 is reversed. As a result, the diode D2 becomes conductive and the zero-voltage switching of the MOSFET Q1 can be achieved.
At this time, as described above, the capacitor C5 is temporarily charged at a voltage close to the output voltage on startup when the high-voltage discharge lamp is turned on. This leads to a problem in that a component having a high withstand voltage must be used, although it is not required during the normal operation.
In the power supply apparatus 10, since the load is light and the output current Iout becomes small when turning on (and before turning on) the high-voltage discharge lamp, the current ib flowing through the resonance coil L2 becomes small. Therefore, an electric charge that is charged on the capacitor C5 in the reversed direction of the normal operation is not discharged, and that state is maintained. In this case, since the voltage Va does not become negative, the direction of the current ib that flows through the resonance coil L2 does not become reversed. Thus, the zero-voltage switching of the MOSFET Q1 cannot be achieved.
Furthermore, since the impedance when turning on the high-voltage discharge lamp is high, as described above, negligible load current flows. In this case, the control circuit 11 makes the ON period of the MOSFET Q1 short in order to prevent the output voltage from becoming greater than or equal to a predetermined value. However, the ON period cannot be shortened without limitation. When the switch element is turned on, the switch element cannot be immediately turned off. Thus, the minimum ON period is determined. This causes excessive power supply and increases the output power voltage. To prevent the excessive power supply and increased output power voltage, the control circuit 11 enters a blocking oscillation mode in which turning on of the MOSFET Q1 is stopped once or several times to suppress the rising of the output voltage.
However, since the frequency and cycle of the switching are fixed, even if the load suddenly changes from light to heavy when the MOSFET Q1 is turned off in the blocking oscillation mode, the MOSFET Q1 cannot be immediately turned on in accordance with the change of the load. As a result, there is a problem in that the output voltage is reduced. This means a long time is required for shifting to a stable turn-on state since the load resistance suddenly becomes small when the power supply apparatus for the high-voltage discharge lamp starts discharging as described above, and the operation cannot follow the change of the load.
Moreover, the state in which the capacitor C5 is charged in the reversed direction of the normal operation is maintained when the load is light on startup, as described above. The charging voltage of the capacitor C5 in this case is, for example, 280 V in the direction in which the MOSFET Q2 side becomes negative. When the load immediately after the high-voltage discharge lamp is turned on has low resistance, the electric charge charged on the capacitor C1 at 280 V is discharged toward the load, and the electric charge charged on the capacitor C5 at 280 V in the reversed direction of the normal operation is also discharged toward the load. This is referred to as a secondary inrush current.
In this case, the discharge current of the capacitor C5 flows through the resistor R1 and the overcurrent protection circuit operates. When the overcurrent protection circuit operates, however, the switching of the power supply apparatus 10 becomes intermittent and this operation is the same as the operation with a light load, and there is a disadvantage in that the operation cannot follow the change of the load thereafter.